1. Field of the Invention
The present invention relates to communicating between transceivers over a common communication channel.
2. Description of the Prior Art
When communicating between two transceivers over metallic conductors, optical fibers, or other media, there often is provided a defined relationship between the transceivers. That is, one of the transceivers is designated as the "master", and the other as the "slave". This designation is useful when prescribing the protocol for establishing communications between the transceivers. For example, the master may send out a pulse or other signal to activate the slave when communications are desired. This allows bi-directional communications over a single channel, by time sharing the channel between the two transceivers. If a suitable protocol were not established, there is the possibility that a transceiver will attempt to transmit while data is incoming, which can result in lost information.
One early technique for allocating communication time slots among stations was the "Aloha net", wherein a station desiring to transmit would broadcast a burst of digital data to a number of other stations. The address of the intended station was included in the transmission, and the transmitting frequency was then monitored by the transmitting station to determine whether another station had transmitted at the same time. If so, the transmission to the desired location was repeated, to ensure that the information was not lost due to interference.
The use of two transceivers in a time-shared manner over a single communication channel is usually referred to as "half-duplex" operation. Transmitting digital signals in time-shared groups is also referred to as the "burst mode" or "ping-pong" technique. It is possible to avoid the time-sharing requirements of these techniques, but at the cost of an additional channel, resulting in "full duplex" operation.
In the half-duplex technique, the slave is maintained in a state receptive to the signal transmitted from the master at least during the approximate time period during which communications are expected. Normally, transmissions from the slave unit are inhibited during this waiting period. For these and possibly other reasons, the master and slave units are required to be differentiated in some manner. This differentiation is accomplished by a change in the characteristics of the transceivers during manufacture or use. For example, in one current half-duplex transceiver implemented in an integrated circuit chip, both the master and slave transceiver chips are initially formed identically. That is, their time bases, transmitter, and receiver portions are initially identical as formed on the chip. However, during a latter phase of the production process, a different metalization pattern designates one unit as the master, and the other as the slave. It is also possible for the user to perform the designation, as by the appropriate connection of an integrated circuit terminal. The designation by whatever technique then causes the slave transceiver to remain in the receiver state until the signaling pulse is received from the master. That is, only the master can initiate communications over the channel.
The prior art master/slave designation has the disadvantage that the flexibility of use of a given transceiver is reduced, since it can perform only as a master or slave after a given designation. Also, if the designation is performed during manufacture, then two different types of transceivers must be manufactured, stocked, and distributed, increasing marketing costs. If the designation is to be performed by the user, additional instructions and labor are required for installation. Furthermore, if the transceivers are part of a switched system employing more than one half duplex channel, then the flexibility in assigning a transceiver to a given channel is reduced, since the appropriate master/slave relationship must be maintained at the ends of the channel. It would clearly be desirable to obtain a half-duplex transceiver that avoids the necessity of the master/slave designation.
Another prior art technique that allows for bidirectional communications over a single channel utilizes a collision detection scheme. For example, the Ethernet (a trademark of Xerox Corp.) networking technique uses transceivers that monitor the common channel for incoming messages and traffic between other units. A given transceiver transmits only when the channel is not busy. It is possible for transmission to begin just as traffic is detected, resulting in a "collision", wherein data from two (or more) transceivers are simultaneously present on the channel. The two (or more) colliding transceivers then wait a random time period before attempting transmission again. If re-transmission is not then successful because of a second collision, the time period is again changed. Note however that in the Ethernet technique each transceiver on the network must still be designated in some manner to specify its address to other units. Furthermore, the collision avoidance technique must be practiced every time a transmission is to begin. Also, relatively sophisticated control circuitry is required to detect and avoid collisions. Hence, this networking technique is overly complicated than is desirable for implementing a low-cost two-transceiver communication link over a single channel.